One of the challenges in the manufacturing of today's complex electronics is verification of correctness of assembly. Electronic components are commonly assembled onto a printed circuit board (PCB). This board is made of substrate layers (commonly of FR4 grade fiberglass) with copper laminated onto the surface of each layer (as few as 2 layers of copper may be used, though often many layers are required). Each of these copper layers is etched using a photographic process, the resulting patterns of copper providing conductive paths on (and through) the PCB. On the outer surfaces of the PCB copper pads are positioned which reflect the layout of the leads (i.e., legs or pins) of the electronic components that will be attached (typically soldered) to the PCB. The resulting assembly is known as a printed circuit board assembly (PCBA).
Configurable fixtures for testing of PCBA have long been known and utilized. The most common variety, known generically as a “bed of nails” fixture, has taken various forms (see, for example, U.S. Pat. Nos. 4,643,501, 5,510,722, 5,450,017 and 6,469,531) and is engageable with any of a variety of known commercially available test system assemblies which provide standardized testing algorithms used to produce electrical stimulus and measure responses of the individual components installed on the PCBA. Provision of full access to every circuit net (i.e., node) of the PCBA thus allows all components to be verified for proper value and function, and such bed of nails fixtures are configured to allow simultaneous engagement of test leads with each selected circuit net of the PCBA under test (often referred to as the UUT, Unit Under Test). Heretofore known and/or utilized fixtures have met the challenge of accurate node contact with as ideal an electrical connection as possible with varying degrees of success.
Node contact pins, known as spring probes, are utilized most commonly in modern test fixtures (see U.S. Pat. Nos. 6,570,399, 4,885,533 and 4,749,945, for example). These probes are positioned at locations in the fixture to allow engagement with the soldered connection of the component leads or other patterns etched on the PCB. The goal of spring probe placements is to have a dedicated probe making contact to every net on the UUT.
Engagement of the spring probes at the UUT has heretofore been accomplished by mechanical clamping, pneumatic cylinders, or vacuum actuated fixturing, the latter currently thought to be the most cost effective. In such fixtures, the UUT is carried atop a moveable plate which is pulled downward upon application of vacuum to a chamber beneath it. The spring probes are located in sockets that are pressed into a second stationary plate and contact the surface of the UUT at locations defined by the topography of the UUT. The sockets are provided with an attachment for wires at the opposite side from the probes (commonly using a wire wrap style connection, though other electrical connection methods are known). The opposite ends of the wires are connected to contacts at an interface configured to correspond to a standard grid of one of the variety of commercially available testers (see U.S. Pat. No. 5,510,722, for example).
Having reference to FIG. 1, a type of vacuum actuated test fixture 12 typical of the prior art for testing UUT's utilizing a standard commercially available tester is shown. These wired fixtures current employ a three plate design. Interface plate 14 is engagable with tester interface 16 of a commercial tester. Probe plate 18 has sockets 20 selectively positioned thereat, sockets 20 receiving probes 22 therein, the probes engaging the selected nodes of the PCBA 24 under test (the UUT). Each probe plate 18 is of necessity configured to accommodate a particular PCBA 24. Diaphragm plate 26 carries PCBA 24. These three plates (14, 18 and 26) define two chambers in prior art fixture 12, upper chamber 28 (the variable volume vacuum chamber, shown in its fully contracted, vacuum-applied state) between the probe plate 18 and diaphragm plate 26 which contracts or expands responsive to application and release, respectively, of vacuum applied in the conventional manner to fixture 12, and lower chamber 30 (the wiring chamber) between interface plate 14 and probe plate 18.
Interface plate 14 (typically a stable FR4 fiberglass material) has a multiplicity of interface wiring terminals 32 which are pressed into thru-holes drilled at standard grid locations corresponding to the tester's matching array of interface pins 34. Terminals 32 are typically wire wrap style terminals. Chamber 30 is thus cluttered with wires 36 extending from terminals 32 to terminals 38 at the opposite side of spring probe sockets 20. Wires 36 are necessary relatively long (300 cm to 1 meter) since access to wiring chamber 30 is required. Hinge 40 connected with the housing holding plates 18/26 allows opening and closing of wiring chamber 30.
Vacuum chamber 28 is provided with small spacers 42 which set the distance between diaphragm plate 26 and probe plate 18 after the chamber is evacuated (providing a small space that keeps the vacuum force applied at diaphragm plate 26). Probe plate 18, diaphragm plate 26, PCBA 24, and the field of spring probes 22 together are called the vacuum head. The vacuum head is not mechanically coupled to any other structure and is hence a stand-alone system. Mechanically, after vacuum chamber 28 is fully evacuated and static, plates 26/18 are subject to flexure, an undesirable consequence that should be minimized (such movement effects probe contact location and stability and thus fixture precision and reliability). Dynamic flexure is dependent on plate thicknesses and the probe field forces.
An alternative to wired test fixtures (such as that shown in FIG. 1) has heretofore been suggested and/or utilized (see U.S. Pat. Nos. 6,628,130, 6,025,729 and 6,066,957). One such wireless fixture, for example, substitutes a double sided printed circuit board for the traditional fixture interface plate engagable at the tester. The circuit board provides conductive traces which make connections from the tester's standard interface to the probes contacting the UUT. These fixtures still require a three plate design (an interface plate that connects to the test system, a probe plate which positions the probes for engagement to the UUT, and a diaphragm plate which is the carrier for the UUT). There still exist two separate chambers, the lower chamber between the interface plate and probe plate and the upper chamber between the probe plate and diaphragm plate. A double-sided spring probe is used to make contact from the interface circuit board to the bottom connection of the probe socket. The upper end of the socket contains a second spring probe which contacts the UUT.
Various other test fixture embodiments have been heretofore suggested and/or utilized that employ translator boards between the interface plate and the probe plate (see U.S. Pat. Nos. 4,935,696, 6,005,405 and 4,884,024, and U.S. Patent Application Publication No. US2003/0030454). These fixtures allow utilization of shorter wiring pathes, but otherwise still require multiple plates (typically four including the interface plate, probe plate, UUT positioning plate and translator plate), as well as multiple different types of probes and sockets.
All of the heretofore known and/or utilized fixtures have suffered to one degree or another from mechanical disadvantages due to the unfettered nature of the vacuum chamber and the relative lack of support for the probe plate and diaphragm plate. Additionally, modifications of probe locations and related interface connections are not easily accommodated by some heretofore known alternatively designed fixtures, since such changes would require that the printed circuit board be redesigned. Other attempts at improvement have merely resulted in overly complex, and thus expensive, designs and modifications, and/or have made the installation of PCBA's, probes, terminals and wiring (where present) burdensome.
Since electronic components continue to shrink in size, fixture design must accommodate target areas for spring probe contacts that also continue to be reduced in size. This in turn requires the mechanical functioning of such fixtures to be more precise and stable. One factor affecting the precision and accuracy probe contact at selected UUT locations is flexure of fixture components subject to vacuum. Another factor is the overall length of the probe itself; a shorter probe is less likely to produce error due to the azimuth angle.
Since ever higher frequencies are utilized by electronic components, it is ever more desirable to keep signal paths in test fixtures as short as possible. This eliminates error due to noise and due to transmission path resistive and reactive impedance losses. The shorter the signal path, the better the testing accuracy.
It can be appreciated, therefor, that further improvement in electronic assembly test fixtures and methods, particularly those improvements directed to accommodating better mechanical functioning and shorter signal paths, could thus still be utilized.